Ferroelectricity in a quasiamorphous ultrathin BaTiO${}_3$ film

Until now, the quasiamorphous (QA) phase in ${\text{BaTiO}}_3$ (BTO), ${\text{SrTiO}}_3$ (STO), and ${\text{BaZrO}}_3$ was achieved by pulling a thick film through a steep temperature gradient. Here, we show that a room-temperature deposited ultrathin film, subsequently annealed in ${\text{O}}_2$ can also produce a QA phase. The atomic, electronic, and ferroelectric (FE) structure of a QA, ultrathin BTO grown on STO were studied by x-ray diffraction (XRD), x-ray photoelectron diffraction (XPD), x-ray photoelectron spectroscopy (XPS), and piezoforce microscopy (PFM). The absence of long-range order is confirmed by in- and out-of-plane XRD as well as Ti 2$p$ XPD. FE polarized domains with good retention have been successfully written into the QA film and exhibit a clear $P-E$ hysteresis loop. Substrate clamping frustrates volume expansion during annealing leading to a QA film. Photoelectron spectroscopy confirms a similar overall electronic structure as for thicker films but with some significant differences. Simple charge-transfer arguments are not sufficient to explain the high-resolution core-level spectra. Ba, Ti, and O all show components associated with a surface region. We suggest that the observation of such a component in the Ti 2$p$ spectrum is linked with the high dynamic charge tensor induced by the large off-center displacement of the Ti ion.

Figure 1

AFM topography image of (a) STO substrate (b) sample 1 (crystalline BTO film) and (c) sample 2 (QA BTO film).

Figure 2

Out-of-plane scan XRD spectra of sample 1 (upper) and sample 2 (lower) subsequently identified as crystalline and quasiamorphous BTO films, respectively. The two peaks located at $2\theta =22.{75}^{\circ }$ and $46.{48}^{\circ }$ are due to STO (001) and STO (002) reflections, respectively. (inset) In-plane scans of crystalline (upper) and quasiamorphous (lower). The STO substrate is at $2\theta =32.{38}^{\circ }$. The low-angle structure in the crystalline film corresponds to fully relaxed BTO, there is a slight low-angle asymmetry in the scan of the quasiamorphous film.

Figure 3

Ti-2$p$ XPD diffraction patterns from (a) crystalline BTO and (b) sample 2 (QA BTO).

Figure 4

PFM phase image of sample 2 (QA BTO film) showing contrast originating from ${\text{P}}^{+}$ and ${\text{P}}^{-}$ polarizations (a) immediately after writing and (b) 24 hours later. (c) Ferroelectric $P-E$ hysteresis loop for the QA BTO film obtained 24 hours after writing. The direction around the loop is indicated by the arrows. (d) Schematic of the distorted ${\text{TiO}}_6$ octahedra in quasiamorphous BTO. The off-center Ti displacement is along $\langle 111\rangle$, indicated by the arrow giving rise to shorter and longer Ti-O bonds.

Figure 5

Core-level spectra for crystalline (top) and QA (bottom) films: (a) C-1$s$, (b) Ti-$\text{2}{\text{p}}_{3/2}$, (c) Ba-$\text{3}{\text{d}}_{5/2}$, (d) O-1$s$, and (e) Sr-3$d$ spectra from the STO substrate under the films.

Figure 6

Valence band spectra of the crystalline and QA BTO. (Inset) Fermi-level region of the crystalline BTO film.